The PRIMO programme (PWR Reference Irradiation of MOX Fuels) was an investigation on MOX fuel, with the following major objectives:

1. Irradiation of MOX fuel rods at different burnup stages and following power histories representative of those of PWR power plants to determine their behaviour as far as their mechanical, thermal and neutronic properties are concerned. The data obtained have been used for the following purposes:
- For licencing, to demonstrate the ability of the MOX fuel rods to sustain irradiation conditions comparable to uranium dioxide fuel;
- To better understand the influence of the fabrication process on the fuel behaviour;
- To benchmark the thermomechanical codes for MOX fuel calculation.
The major phenomena to be examined were:
- Mechanical behaviour under steady-state operation: pellet-cladding interaction, ridging effects, fuel swelling and creep
- Thermal behaviour: fission gas release, grain growth, fuel diffusion effects
- Nuclear behaviour: burnup radial profiles and U/Pu isotopic effects, versus burnup

2. Execution of a ramp test programme, to find out the failure threshold of MOX fuel rods and to obtain mechanical and thermal data under ramp conditions. The ramp tests were carried out on selected fuel types, at various burnup levels. The ramp conditions (ramp rate, terminal power and hold time) were chosen such that the results can be compared to existing results on UO2 fuel.

3. Fast power transients on a MOX rod to simulate a class II incident in a PWR.

These objectives were met through extensive post-irradiation examinations (PIE) in hot cells.

The base programme includes different 'types' of fuels.

The data provided in this compilation concerns the BD8 fuel rod manufactured at Belgonucleaire, using the MIMAS (Micronized Master Blend) process presently used for industrial production of LWR MOX fuel during the first BR3/4D MIMAS campaign (1983). The use of the Master blend principle leads to fuel presenting some Pu-rich particles, which can locally reach the content of ~30% PuO2 of the primary blend, depending on the size of the agglomerates and on the U-Pu interdiffusion during sintering.

Rod BD8 was irradiated in the BR3 SCK-CEN reactor during cycles 4D1 and 4D2 up to a peak burnup of 30.1 GWd/tM prior to being transferred to the ISOBELLE 1 loop for ramping in OSIRIS.

During base irradiation, the BR3 reactor nominal power was 40.9 MWth (11.4 MWe). The primary circuit operating pressure was 140 atm. The average temperature of the coolant was maintained at 262.5 deg.C (power changes were compensated by adapting the mass flow rate). Coolant inlet temperature was 255 deg.C, and the mean outlet temperature at nominal power reached 270 deg.C.

The programme PRIMO was started in October 1986. It was jointly organized by SCK-CEN (Studiecentrum voor Kernenergie - Centre d'Etudes de L'Energie Nucleaire) and BN (BELGONUCLEAIRE) and was co-sponsored by ten participants including Fuel Vendors, Utilities, Nuclear Centers and National Authorities.